The present invention relates to a system for maintaining power consumption below a predetermined level and, more particularly, to an add/shed control for maintaining power consumption below a predetermined level where load power demand can be partially increased or decreased in different sequences depending upon the prioritization level given to each partial load upon the increase/decrease sequence assigned to each prioritization level.
Power utilities charge their customers according to a policy which is designed to both encourage energy conservation and to assess the cost for the extra power generating equipment which they must acquire and maintain to meet peak demands against those creating the peak demand. Accordingly, these power utilities will typically charge their customers at a first rate for power consumed below a first predetermined level, at a second rate the power consumed between the first predetermined level and a second predetermined level. If power consumption should exceed the second predetermined level, the utilities will charge what is in effect a penalty because they have had to acquire and maintain extra power generating equipment to meet those periods of unusually high demands. If all of utilities customers could be encouraged to eliminate excessive usage of power, the power utilities would then not be required to provide the extra equipment to generate the extra power during these peak demand periods and would be able presumably to pass along these savings to its customers through lower charges. In order to encourage economies and power consumption by their customers, the public utilities therefore, charging "penalties" on those customers who create peak demands.
In order to avoid these peak demand charges imposed upon them by their power utilities, power customers have employed automatic control systems which monitor power consumption within their facilities and then control the energization state of the power consuming loads within their facilities to maintain power consumption below some predetermined value in order to reduce the extra charges which would be imposed upon them if their energy usage increased the peak usage over a given time interval. These systems have typically taken the form of add/shed control systems which are designed to shed loads as power consumption approaches a predetermined level which is established by the customer. Alternatively, as power consumption begins to fall away from this predetermined level, previously shed loads can be added back on line so that they may be utilized by the customer.
Several different types of add/shed control systems have been proposed in the past. For example, the most common type of add/shed control system establishes a prioritized load order wherein the load having lowest priority will be shed first and the load having highest priority will be shed last. In such a system, if loads can be added back on line, the load having the highest priority will be added first and the load having the lowest priority will be added last. This is commonly referred to as a sequential routine.
Another add/shed system which is known adds/sheds loads on the basis of comfort fairness. That is, the loads associated with zones having conditions closest to set point will have the lowest priority and loads associated with zones having conditions the farthest away from the set point will have the highest priority. In this system, the loads having lowest priority will be shed first and the loads having highest priority will be shed last on the basis that it is more fair to shed the loads associated with the zones being closest to the comfort range first. Unfortunately, these systems have been inflexible because rarely can all the loads within a building be made to fit within one of these types of add/shed routines. For example, it is more desirable to shed the air handling fan systems within the building according to a comfort fairness routine than according to a sequential routine because a sequential routine is a fixed routine and will not necessarily result in fair control of the air handling fan systems. On the other hand, the outdoor lights certainly would not be shut on the basis of comfort fairness and it may also be determined that the outdoor lights are all of equal priority so that they could not be prioritized according to a sequential routine. Thus the outdoor lights may be shed using a rotational routine so that the first shed will be the first added.
As a result the new and improved add/shed control was developed by the assignee of the present invention and is fully disclosed in U.S. Pat. No. 4,510,398 which is assigned to the assignee of the present invention. In this system, an add/shed control is provided for maintaining power consumption below a predetermined level where the loads can be added and/or shed in different sequences depending upon the prioritization level given to each load and upon the add/shed sequence assigned to each prioritization level. Of the add/shed control disclosed in the aforementioned U.S. Pat. No. 4,510,398 adds flexibility to such systems, there remains substantial room for improvement. For example, the foregoing system does not provide for determining the anticipated future increase or decrease and power consumption of the building and as a result is unable to predict which loads must be shed in order to maintain the power consumption of a building below a predetermined level when future loads are to come on line. Hence, all of the add/shed control systems previously described are reactive systems which can only react to existing circumstances and are unable to maintain power consumption of a building below a predetermined level on a continuous basis based upon known future increases or decreases in power consumption.
In addition to the foregoing, such systems usually include a plurality of loads which are under control of "applications packages" which control these loads based upon a predetermined on and off schedule. When one of the loads assigned to an application package is to come on line, the system can only react to the load coming on line after it is on and drawing power which may cause the power consumption of the building to instantaneously exceed the desired predetermined level.
In order to overcome deficiencies noted in the prior art system, the system taught by U.S. Pat. No. 4,916,328 assigned to the assignee of the present invention, was created. The system taught in the '328 patent includes a means and method for determining the anticipated future increase or decrease in power consumption of the building. Selected loads are then added or shed in response to the measured total power consumption of the building and the anticipated future increase or decrease in power consumption of the building thereby continuously maintaining the power consumption of the building below the predetermined second level.
With the inclusion of multispeed fans and other analog loads to building automation systems, it is no longer necessary to shed entire loads to maintain power consumption below the second predetermined level. It is therefore an object of the present invention to maintain energy usage in a building below the second predetermined level by partly shedding loads which have multiple operational states.